I was hoping somebody would ask. In fact just to beg the question I specifically searched for '74 Suburban pics of both types and luckily found them from the same eBay seller, even taken in the same position. So this is what's up - from the 1974 Schwinn Dealer News Flash #10 dated 3/28/74:
I'm a believer of not waiting for people to ask, just throw it out there and we'll talk about. I think it's OK to ask and answer your own questions online. Anything to get the conversation going. You never know where it might lead. You mentioned to me about year appropriate reflectors, I'd like to read about that.